Central Vacancy Research Articles

Graphene-supported MB36 clusters (M=Fe, Co, Ni) as a single-atom electrocatalyst for the oxygen reduction reaction: A DFT study

This study delves into the potential of MB36 clusters (where M = Fe, Co, Ni) supported on graphene as effective single-atom catalysts (SACs). Our research is methodically segmented into several stages, each designed to validate the feasibility of the proposed material. We begin by investigating the potential energy surface of MB36 clusters. Our findings reveal a preference for metallic atoms to position themselves above the central hexagonal vacancy of the B36 cluster. This factor significantly contributes to the stabilization of the resultant MB36 system. Subsequently, we employ ab-initio molecular dynamic simulations to affirm the stability of the graphene-supported MB36 clusters. In the final stage, we evaluate the system's catalytic capability for the oxygen reduction reaction (ORR) by calculating the dissociation energy of O2 and OOH post their adsorption on the M atom of the graphene-supported MB36 cluster. These simulations suggest that FeB36 could maintain stability at room temperature, exhibiting activation energies for O2 and OOH dissociation lower than those previously reported on Fe/N/C-type SACs, approaching those found on the Pt(111) surface, the most efficient ORR electrocatalyst. This computational study suggests that graphene-supported FeB36 clusters could emerge as a promising candidate for SACs.

Quantifying the impacts of suburbanization without growth on central city housing vacancy

AbstractScholars have written descriptively about the relationship between suburbanization and central city decline. However, research has not produced systematic, quantitative estimates of the relationships between the components of suburbanization and vacancy—generally or amid urban decline. Such estimates are required to design effective policies that could protect against the social inequalities associated with such development patterns. This paper applies an economic framework of suburbanization and vacancy to 99 urbanized areas in the United States to estimate the impact on central city residential vacancy from 2013 to 2019. The results support the central hypothesis, that regional over‐building contributes to central city vacancy, and that this impact is significantly magnified by the context of decline. Results show regional over‐building causes central city vacancy in Shrinking Cities at 9.6 times the effect observed in other Principal Cities. Ameliorating factors vary between Shrinking and Non‐Shrinking contexts.

Central nitrogen vacancies in polymeric carbon nitride for boosted photocatalytic H2O2 production

Polymeric carbon nitride (PCN) with vacancies usually exhibits distinguished mass transfer efficiency, outstanding carrier kinetics and excellent photoactivity. Previous studies have revealed the effect of edge vacancies in heptazine units of PCN; however, the roles of central nitrogen vacancies are scarcely investigated. Herein, central nitrogen vacancies polymeric carbon nitride (PCN-NVC) is rationally prepared for photocatalytic H2O2 production with a rate of 25.1 umol/h (λ > 420 nm), which is 3.5 times than that of pristine PCN. Photoelectronic measurements reveal that the central nitrogen vacancies optimize the kinetic process of electron-hole pairs. Density functional theory (DFT) calculations disclose that PCN-NVC displays lower O2 adsorption energy, thereby accelerating the OOH* formation and decreasing the H2O2 generation energy barrier. This work not only provides a strategy for constructing central nitrogen vacancies polymeric carbon nitrogen, but also affords a deep understanding of its roles in photocatalytic H2O2 production.

Reductive dehalogenation in groundwater by Si-Fe(II) co-precipitates enhanced by internal electric field and low vacancy concentrations

Fe(II) and silicate can form Si-Fe(II) co-precipitates in anoxic groundwater and sediments, but their phase composition and reactivity towards subsurface pollutants are largely unknown. Three types of Si-Fe(II) co-precipitations with the same chemical composition, namely Si-Fe(II)-I, Si-Fe(II)-II, and Si-Fe(II)-III, have been synthesized by different hydroxylation sequences in this work. It was found that Si-Fe(II)-III reduce carbon tetrachloride (CT) much faster (k1=0.04419 min−1) than Si-Fe(II)-I (0 min−1) and Si-Fe(II)-II (7.860 × 10−4 min−1). XRD results show that the main component of Si-Fe(II)-III is ferrous silicate (FeSiO3), which is quite different from that of Si-Fe(II)-I and Si-Fe(II)-II. The unique arrangement of hydroxyl coordination, the less distorted octahedral structure, the polyhedral morphology and the absence of Si-A center vacancies in Si-Fe(II)-III are responsible for its high reductive dehalogenation reactivity. The highest redox activity of Si-Fe(II)-III was shown by electrochemical characterization. The [FeII-O-Si]+ in Si-Fe(II)-III may stabilize the dichlorocarbene anion (˸CCl2−), which favors the transformation of CT to methane (9.2%). The Si-Fe(II) co-precipitates consist of countless internal electric fields, and the transformation of hydroxyl and CT both consumed electrons. The coexistence of hydroxyl and CT increases the electron density in the electron-rich region due to their electronegativity, enhancing their electron-accepting capabilities. This study deepens our understanding of the phase composition and electronic structure of Si-Fe(II) co-precipitates, which fills the gap in the reductive dehalogenation of halides by Si-Fe(II) co-precipitates.

Growth Pattern of Large Morse Clusters with Medium-Range Potentials.

Due to the extremely complex potential energy surfaces of large Morse clusters with medium-range potentials (i.e., ρ = 6 and 10), global optimization studies in the literature are limited to a cluster size (N) of ≤240. Starting from completely random structures, we successfully systematically studied Morse clusters with up to 700 atoms using our unbiased fuzzy global optimization (FGO) method. While all of the putative global minima reported previously have been efficiently obtained, new global minima with lower energies are identified for N values of 176, 258, 485, 561, 817, and 923 with ρ = 6 and for N values of 151, 202, 226, and 229 with ρ = 10. A detailed growth pattern and magic clusters are obtained. For the first time, we find that a central vacancy is present in Morse clusters containing 542, 543, 548, and 922 atoms with ρ = 6. FGO has achieved high performance in large clusters with different interatomic interaction ranges, thus showing great application potential in the global structure optimization of general clusters.

Unusual local electric field concentration in multilayer ceramic capacitors

Local electric-field around multitype pores (dielectric pore, interface pore, electrode pore) in multilayer ceramic capacitors (MLCCs) was investigated using Kelvin probe force microscopy combined with the finite element simulation to understand the effect of pores on the electric reliability of MLCCs. Electric-field is found to be concentrated significantly in the vicinity of these pores and the strength of the local electric-field is 1.5–5.0 times of the nominal strength. Unexpectedly, the concentration degree of the pores in the inner electrode is much higher than that in the dielectrics and dielectric-electrode interfaces. Meanwhile, geometry orientations are found to have a remarkable influence on the local electric field strength. The pores act as an insulation degradation precursor via local electric, thermal center, and oxygen vacancies accumulation center. Such unusual local electric field concentration of multitype pores can provide new insights into the understanding of insulation degradation evolution, processing tailoring and design optimization for MLCCs.

Reassignment of magic numbers for icosahedral Au clusters: 310, 564, 928 and 1426.

Icosahedral Au clusters with three and four shells of atoms are found to deviate significantly from the commonly assumed Mackay structures. By introducing additional atoms in the surface shell and creating a vacancy in the center of the cluster, the calculated energy per atom can be lowered significantly, according to several different descriptions of the interatomic interaction. Analogous icosahedral structures with five and six shells of atoms are generated using the same structural motifs and are similarly found to be more stable than Mackay icosahedra. The lowest energy per atom obtained here is for clusters containing 310, 564, 928 and 1426 atoms, as compared with the commonly assumed magic numbers of 309, 561, 923 and 1415. Some of the vertices in the optimized clusters have a hexagonal ring of atoms, rather than a pentagon, with the vertex atom missing. An inner shell atom in some cases moves outwards by more than an Ångström into the surface shell at such a vertex site. This feature, as well as the wide distribution of nearest-neighbor distances in the surface layer, can strongly influence the properties of icosahedral clusters, for example catalytic activity. The structural optimization is initially carried out using the GOUST method with atomic forces estimated with the EMT empirical potential function, but the atomic coordinates are then refined by minimization using electron density functional theory (DFT) or Gaussian approximation potential (GAP). A single energy barrier is found to separate the Mackay icosahedron from a lower energy structure where a string of atoms moves outwards in a concerted manner from the center so as to create a central vacancy while placing an additional atom in the surface shell.

Central vacancy creation in icosahedral nanoparticles induced by the displacement of large impurities

We employ metadynamics simulations at room temperature to study the diffusion of large single-atom impurities within otherwise pure icosahedral nanoparticles, for different bimetallic systems (Au-Co, Ag-Co, Ag-Ni, Au-Pt and Au-Rh) and icosahedral sizes. Our simulations reveal that the displacement of the impurity induces the formation of a vacancy in the central part of the structure, as already observed for small icosahedra of Au-Co and Ag-Cu, therefore confirming the generality of this peculiar diffusion process. At the largest size, a new collective displacement mechanism is identified alongside the diffusion of the impurity. For Au-Pt clusters, different diffusion pathways are observed, which are mediated by a partial deformation of the nanoparticle surface. This is due to the lack of stability of fivefold veritices in Pt icosahedral clusters. We prove that the stability of the icosahedral surface is an essential condition for the occurrence of the combined process of impurity diffusion and internal vacancy formation.

Primary radiation damage in silicon from the viewpoint of a machine learning interatomic potential

Characterization of the primary damage is the starting point in describing and predicting the irradiation-induced damage in materials. So far, primary damage has been described by traditional interatomic potentials in molecular dynamics simulations. Here, we employ a Gaussian approximation machine-learning potential (GAP) to study the primary damage in silicon with close to ab initio precision level. We report detailed analysis of cascade simulations derived from our modified Si GAP, which has already shown its reliability for simulating radiation damage in silicon. Major differences in the picture of primary damage predicted by machine-learning potential compared to classical potentials are atomic mixing, defect state at the heat spike phase, defect clustering, and recrystallization rate. Atomic mixing is higher in the GAP description by a factor of two. GAP shows considerably higher number of coordination defects at the heat spike phase and the number of displaced atoms is noticeably greater in GAP. Surviving defects are dominantly isolated defects and small clusters, rather than large clusters, in GAP's prediction. The pattern by which the cascades are evolving is also different in GAP, having more expanded form compared to the locally compact form with classical potentials. Moreover, recovery of the generated defects at the heat spike phase take places with higher efficiency in GAP. We also provide the attributes of the new defect cluster that we had introduced in our previous study. A cluster of four defects, in which a central vacancy is surrounded by three split interstitials, where the surrounding atoms are all 4-folded bonded. The cluster shows higher occurrence in simulations with the GAP potential. The formation energy of the defect is 5.57 eV and it remains stable up to 700 K, at least for 30 ps. The Arrhenius equation predicts the lifetime of the cluster to be $0.0725\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{s}$ at room temperature.

Fe decorated CeO2 microsphere catalyst with surface oxygen defect for NO reduction by CO

The selective catalytic reduction of nitrogen oxides (NOx) by carbon monoxide (CO-SCR) is a promising way to deal with exhaust gasses. And Effective redox cycle and NO dissociation ability are crucial factors for catalytic reduction of NOx. Herein, Fe decorated CeO2 microsphere catalysts with surface oxygen defect were synthesized by solvothermal and calcination method. Among all catalysts, 0.5Fe-CeO2 sample showed excellent catalytic performance with efficiency near 96% for NO reduction under the condition of 550 ppmv NO, 1200 ppmv CO, and 40,000 h − 1 of GHSV. It was found that Fe doping could efficiently regulate Ce4+/Ce3+ redox couple and oxygen vacancy for catalytic activity. XRD, Raman, XPS, EPR and H2-TPR etc. were employed to explore the physicochemical properties of samples. The results showed that Fe had been incorporated into CeO2 by XRD, and the results of Raman spectrum and XPS indicated that Ce4+/Ce3+ redox couple existed in 0.5Fe-CeO2 catalyst. Furthermore, the H2-TPR spectrum displayed the formation of oxygen vacancy in the samples. N2 adsorption-desorption isotherms indicated that the doping of Fe could enlarge the specific surface area, pore volume and pore dimeter of the samples, and thus greatly enhanced the NO adsorption and further NO catalytic reduction on catalysts. In addition, other characterization of physicochemical properties and density functional theory (DFT) calculation were performed to reveal the mechanism of NO + CO reaction. As a result, a mechanism was proposed to elucidate highly efficient NO reduction over the catalysts of Fe decorated CeO2 microspheres. Finally, the synergistic effects of redox center and oxygen vacancy on catalytic activity enhancement were revealed. This work provides a promising way to the design of efficient catalysts for NO reduction.

DFT Study of α-Keggin-type Iso-polyoxotungstate Anions [HnW12O40](8-n)- (n =1-4): Can [H4W12O40]4- Exist?

The Keggin-type iso-polyoxotungstate (iso-POT) anions [HnW12O40](8-n)- 's, in which their central vacancies are occupied by protons, are attractive materials. It is of importance to reveal if the vacancies can be fully occupied by four protons. For further understanding the speciation of these iso-POT anions, relative stabilities and proton transfer reactions between H1[Hn-1W12O40](8-n)- and [HnW12O40](8-n)- were examined in detail by using the first-principles calculations (the nudged elastic band method, the synchronous transit-guided quasi-Newton method, the intrinsic reaction coordinate method, and frequency analysis calculations). Thermochemistry analysis of the proton transfer was also performed. [HnW12O40](8-n)- was energetically more stable than H1[Hn-1W12O40](8-n)-. This held for n = 4. According to the results of thermochemistry analysis, the rate constant and the Wigner correction were respectively 3.1 × 101 s-1 and 2.2 at T = 298.15 K for the proton transfer from H1[H3W12O40]4- to [H4W12O40]4-, indicating that [H4W12O40]4- can exist when H1[H3W12O40]4- is formed by protonating [H3W12O40]5-.

Shaping Up Zn-Doped Magnetite Nanoparticles from Mono- and Bimetallic Oleates: The Impact of Zn Content, Fe Vacancies, and Morphology on Magnetic Hyperthermia Performance.

The currently existing magnetic hyperthermia treatments usually need to employ very large doses of magnetic nanoparticles (MNPs) and/or excessively high excitation conditions (H × f > 1010 A/m s) to reach the therapeutic temperature range that triggers cancer cell death. To make this anticancer therapy truly minimally invasive, it is crucial the development of improved chemical routes that give rise to monodisperse MNPs with high saturation magnetization and negligible dipolar interactions. Herein, we present an innovative chemical route to synthesize Zn-doped magnetite NPs based on the thermolysis of two kinds of organometallic precursors: (i) a mixture of two monometallic oleates (FeOl + ZnOl), and (ii) a bimetallic iron-zinc oleate (Fe3–yZnyOl). These approaches have allowed tailoring the size (10–50 nm), morphology (spherical, cubic, and cuboctahedral), and zinc content (ZnxFe3–xO4, 0.05 < x < 0.25) of MNPs with high saturation magnetization (≥90 Am2/kg at RT). The oxidation state and the local symmetry of Zn2+ and Fe2+/3+ cations have been investigated by means of X-ray absorption near-edge structure (XANES) spectroscopy, while the Fe center distribution and vacancies within the ferrite lattice have been examined in detail through Mössbauer spectroscopy, which has led to an accurate determination of the stoichiometry in each sample. To achieve good biocompatibility and colloidal stability in physiological conditions, the ZnxFe3–xO4 NPs have been coated with high-molecular-weight poly(ethylene glycol) (PEG). The magnetothermal efficiency of ZnxFe3–xO4@PEG samples has been systematically analyzed in terms of composition, size, and morphology, making use of the latest-generation AC magnetometer that is able to reach 90 mT. The heating capacity of Zn0.06Fe2.94O4 cuboctahedrons of 25 nm reaches a maximum value of 3652 W/g (at 40 kA/m and 605 kHz), but most importantly, they reach a highly satisfactory value (600 W/g) under strict safety excitation conditions (at 36 kA/m and 125 kHz). Additionally, the excellent heating power of the system is kept identical both immobilized in agar and in the cellular environment, proving the great potential and reliability of this platform for magnetic hyperthermia therapies.

LAW, DEVELOPMENT AND ACCESS TO EDUCATION: A BRAZILIAN CASE STUDY OF CLASS ACTIONS

In determining its universal development agenda, to be implemented between 2016 and 2030, the United Nations established 17 Sustainable Development Goals (SDG). Among these, SDG 16 aims to “promote peaceful and inclusive societies for sustainable development, provide access to justice for all and build effective, accountable and inclusive institutions at all levels.” Nevertheless, as the failure of the World Bank-inspired “rule of law” reform efforts demonstrates, achieving this laudable goal in extremely unequal countries is a Herculean challenge. I believe, however, that class action could contribute in a decisive way by (i) benefiting all parties interested in a dispute’s outcome, including those groups systematically excluded; (ii) discouraging unfair access to scarce resources; (iii) helping to deter illicit conduct; (iv) presenting demands in one voice, thus economizing public resources and stimulating faster judgments; and (v) contributing to a level playing field for plaintiff and defendant. By presenting an in-depth case study of a landmark class-action litigation for day-care center vacancies in Sao Paulo, I intend to highlight this legal tool’s strengths and weaknesses and to propose a better institutional design.

Enhanced performance of in-plane transition metal dichalcogenides monolayers by configuring local atomic structures

The intrinsic activity of in-plane chalcogen atoms plays a significant role in the catalytic performance of transition metal dichalcogenides (TMDs). A rational modulation of the local configurations is essential to activating the in-plane chalcogen atoms but restricted by the high energy barrier to break the in-plane TM-X (X = chalcogen) bonds. Here, we theoretically design and experimentally realize the tuning of local configurations. The electron transfer capacity of local configurations is used to screen suitable TMDs materials for hydrogen evolution reaction (HER). Among various configurations, the triangular-shape cobalt atom cluster with a central sulfur vacancy (3CoMo-VS) renders the distinct electrocatalytic performance of MoS2 with much reduced overpotential and Tafel slope. The present study sheds light on deeper understanding of atomic-scale local configuration in TMDs and a methodology to boost the intrinsic activity of chalcogen atoms.

Density functional theory based studies on the adsorption of rare-earth ions from hydrated nitrate salt solutions on g-C3N4 monolayer surface

This article represents density functional theory (DFT) based comparative analysis on six trivalent rare-earth ions (RE3+; RE: Y, La, Ce, Sm, Eu and Gd) absorption, from the respective nitrate-hexahydrate salts, on graphitic carbon nitride (g-C3N4) 2D monolayer, and the photocatalytic properties of the RE3+ adsorbed g-C3N4 systems (g-C3N4/RE3+) based on the ground-state electronic structure calculations. Structure, stability and coordination chemistry of two configurations of each hydrated RE-salt system are discussed in detail. Both DFT (B3LYP/SDD) and semi-empirical (Sparkle/PM7) calculations identify the central N6 vacancy of pristine g-C3N4 as the most suitable site for RE3+ adsorption. Bader’s QTAIM, Mayer bond order and charge population analyses (ADCH, CHELPG and DDEC) are performed to describe the bond characteristics within the systems under study. Thermochemical calculations suggest that the adsorption process is thermodynamically more feasible for higher atomic number (Z) RE3+ [Sm3+, Eu3+ and Gd3+], compared to lower-Z RE3+ [Y3+, La3+ and Ce3+] ions. Besides, the better photocatalytic properties of higher-Z RE3+ adsorbed g-C3N4 systems are revealed from better HOMO-LUMO delocalization, decreased HOMO-LUMO gap, increased softness, higher electrophilicity and electron transfer parameter, compared to pristine or lower-Z RE3+ adsorbed g-C3N4 systems, as obtained from Hirshfeld orbital compositions, density of states and condensed Fukui function analyses.

Strong thermal conductivity dependence on arsenic-vacancy complex formation in arsenic-doped silicon

High-concentration doping of silicon (Si)-based materials is an effective way to improve their thermoelectric efficiency via thermal conductivity (κ) reduction as well as thermopower and electrical conductivity enhancement. Beyond the solubility limit, a large portion of dopant atoms may undergo clustering mediated primarily by point defects, which may in turn significantly alter the physical properties of host materials. In this work, we investigate the effect of Arsenic-vacancy (AsV) complex formation on κ in heavily As-doped Si using molecular dynamics simulations. The simulation results clearly demonstrate that the presence of AsV complexes, particularly As4V which is the most stable one, may result in a substantially lower κ compared to the case where all As atoms remain substitutional and electrically active. Further analysis reveals that the central vacancy in As4V causes Si lattice softening and thus results in the reduction of phonon group velocity. Our findings highlight the significant effect of dopant clustering on κ and also provide some guidance on how to manipulate Si-based materials to improve their thermoelectric performance via doping combined with defect engineering.

B31- and B32-: chiral quasi-planar boron clusters.

Chirality plays an important role in nature. Nanoclusters can also exhibit chiral properties. We report herein a joint experimental and theoretical investigation on the geometric and electronic structures of B31- and B32- clusters, using photoelectron spectroscopy in combination with first-principles calculations. Two degenerate quasi-planar chiral C1 enantiomers (I and II, 1A) with a central hexagonal vacancy are identified as the global minima of B31-. For B32-, two degenerate boat-like quasi-planar chiral C2 structures (VI and VII, 2A) with a central hexagonal vacancy are also found as the global minima, with a low-lying chair-like Ci B32- (VIII, 2Au) also present in the experiment as a minor isomer. The chiral conversions in quasi-planar B31- and B32- clusters are investigated and relatively low barriers are found due to the high flexibility of these monolayer clusters, which feature multiple delocalized σ and π bonds over buckled molecular surfaces.

Magnetization, the susceptibilities and the hysteresis loops of a borophene structure

The magnetic properties of borophene structure are studied for the first time by the effective-field theory with correlations. This method has been successfully used in analyzing the magnetic mechanism of graphene. The borophene consisting of 36 boron atoms with a central hexagonal vacancy can be described by Ising model. The general formula of the magnetization for the borophene is given. Possible physical mechanism of the emergence magnetic properties is demonstrated. The effects of the exchange coupling and the anisotropy of the magnetization, the susceptibility the blocking temperature and hysteresis loops of borophene are discussed. We also illustrate the possibility to test the predicted magnetism in an experiment, such as the maximum on the magnetization curve. Our study may provide an opportunity for the design of a new type of memory-switching or sensors by using earth-rich nonmagnetic boron materials.

Access to the City Center and Housing Vacancies in the Suburbs

How does access to the center of a city affect the housing vacancy rate in the suburbs? Using data on housing for French municipalities, I first document the disparities and evolution of housing vacancy rates in France. A particular attention is paid on the Paris metropolitan area, where housing vacancies are surprisingly large. Next, I develop a theoretical model which emphasizes the link between proximity to the city center and housing vacancies in suburban locations. Finally, I empirically test the predictions of the model using information on transport and housing in the Paris urban area. The findings show that proximity to the city is associated with lower vacancy rates. This result is consistent with a positive association between distance and frictions in the real estate market.

Theoretical Analysis of Pseudodegenerate Zero-Energy Modes in Vacancy-Centered Hexagonal Armchair Nanographene

Deriving mathematical expressions of two zero modes for a $\pi$-band tight-binding model, we identify a class of bipartite graphs having the same number of subgraph sites, where each graph represents one of the quasi-hexagonal nanographene molecule with a center vacancy (VANG). Indeed, in a VANG molecule, C$_{60}$H$_{24}$, showing stability in a density-functional simulation at the highest occupied level, there appear two pseudo-degenerate zero modes, a vacancy-centered quasi-localized zero mode, and extending zero mode with a $\sqrt{3} \times \sqrt{3}$ structure. Since there is a finite energy gap between these two zero-energy modes and the other modes, low-lying states composed of quasi-degenerate zero modes appear as magnetic multiplets. Thus, the unique magnetic characteristics derived in our theory are expected to hold for synthesized VANG molecules in reality.